The present invention relates to the use of a cold flow improver in a “hard-to-treat” fuel.
Generally, distillate fuels are comprised of a mixture of hydrocarbons including normal (linear) and branched-chain (iso-) paraffins, olefins, aromatics and other polar and non-polar compounds, and cold flow behavior is a function of the relative proportion of these various hydrocarbon components. Normal paraffins typically have the lowest solubility and therefore tend to be the first solids to separate from the fuel as the temperature is decreased. At first, individual paraffin crystals will appear but as more crystals form they will ultimately create a gel-like network which inhibits flow. The compositional makeup of fuels can vary widely depending on the crude oil source and how deeply the refiner cuts into the crude oil. Refiners increasingly produce distillate fuels with amounts and types of hydrocarbon components which render the fuels unresponsive to additives which were before capable of imparting acceptable cold flow properties to the fuels (so-called “hard-to-treat” fuels). New groups of additives have been developed for treating such fuels. For middle distillate fuels the most important cold flow improver type is generally described as a middle distillate flow improver (MDFI). This additive type delivers an operability related response measured by CFPP (Cold Filter Plugging Point), which temperature is a parameter that is regulated in some major diesel fuel specifications (such as CEN EN590) or alternative laboratory filterability tests.
With the introduction of Fischer-Tropsch derived fuels (also called Gas-To-Liquid fuels or GTL fuels), which essentially contain paraffinic components, with a relatively high level of n-paraffin species, a new group of “hard-to-treat” fuels became available. Fischer-Tropsch derived fuels are the reaction products of the Fischer-Tropsch methane condensation processes, for example the process known as Shell Middle Distillate Synthesis (van der Burgt et al, “The Shell Middle Distillate Synthesis Process”, paper delivered at the 5th Synfuels Worldwide Symposium, Washington D.C., November 1985; see also the November 1989 publication of the same title from Shell International Petroleum Company Ltd, London, UK). Although MDFI's are available for treating conventional hard-to-treat fuels, it was found that neat (essentially non-blended) Fischer-Tropsch derived (middle distillate) fuels have different properties than the conventional hard-to-treat middle distillate fuels and are generally not responsive to known MDFI's.